In the last few years a number of satellite communication systems have been proposed, built and operated. A particular variety of satellite communication systems which has proved to be economically attractive employs time division multiple access (TDMA). In this system the plurality of earth stations transmit information in burst form (i.e., data bursts) to the satellite for re-transmission to other earth stations. In order to allow a plurality of earth stations to transmit substantially simultaneously, the system is designed to allow each of the transmitting earth stations to transmit in a predetermined period of time such that the respective transmissions will be received in the sequential relationship at the satellite. So long as the respective transmissions are so received effective communication is achieved inasmuch as the satellite then retransmits the information it receives to other earth stations. An essential requirement for TDMA systems is proper synchronization. Loss of synchronization can, and generally will result in transmissions which overlap in time at the satellite. Those of ordinary skill in the art will understand that such a condition will result in garbling of the overlapping transmissions and render the satellite unable to retransmit the information transmitted to it.
In order to synchronize the TDMA systems a single station is designated as a reference station and, in addition to transmitting whatever information it has to transmit, it also transmits a synchronization signal, or sync burst. This sync burst is received at the satellite and retransmitted to all other stations in the same manner as the information received by the satellite is transmitted. Receipt at any earth station of the sync burst allows that station to time its transmission from the sync burst for proper synchronization. A typical example of such a synchronization system is disclosed in Gabbard U.S. Pat. No. 3,562,432. As indicated in that patent proper synchronization is so important that if, at any station, the synchronization burst is not received for "a predetermined time, the station transmitter is automatically disabled to prevent jamming of other bursts".
Of course, there are a number of reasons why a particular earth station will fail to receive the sync burst. If the failure to receive the sync burst is related to a failure at the receiving station then the action taken, i.e., shutting down the receiving stations transmitter, is the most effective action that can be taken to overcome the problem. This possibility is usually remote since the majority of satellite terminals have redundant equipment to minimize such instances. In addition at least one reason for failing to receive the sync burst is that the station transmitting the sync burst has failed to transmit it. If this occurs, of course, each of the stations in the communication system will fail to receive the sync burst and, therefore, each of them will be automatically shut down thereby disabling the entire system. Obviously, this is not a desirable alternative.
Maillet, in U.S. Pat. No. 3,878,339 proposes another alternative to the just mentioned situation. In this apparatus each of the stations monitor their reception of the sync burst. If the apparatus indicates that the sync burst is lost each station begins timing out a preset period. Furthermore, each station receives an indication, from each of the other stations, as to whether or not that other station has also lost the sync burst. If a majority of the stations are indicating a loss of sync burst for the predetermined period of time, then a predetermined one of the remaining stations will then put up a new sync burst. The reason for requiring a plurality of stations to agree on loss of sync burst should be apparent, i.e., to prevent this possibility that a local failure has caused a loss of sync burst indication when, in fact, the reference station is transmitting the sync burst. If that condition is not prevented, it is likely that two stations would put up a sync burst at different times which could result in confusion and prevent effective operation. Inasmuch as the stations are geographically widely separated and in order to provide each station with an indication of whether or not each of the other stations is receiving the sync burst, a signalling channel must be provided between the stations for this purpose.
Although there is no reason to believe that this system would not work, it should be apparent that use of this system will decrease efficiency by requiring a channel so that each station can indicate to each of the other stations whether or not it is receiving the sync burst. Futhermore, additional apparatus is required to detect reception of the sync burst and provide for a proper signalling, to receive other station signalling and to perform the necessary logical operations inherent in employing this system. Furthermore, this system requires that the network operate without a reference sync burst during the replacement cycle for intervals greater than one round trip satellite propagation delays. This requirement places severe constraints on stability and absolute accuracy of satellite terminal frequency standards.
It is therefore one object of the present invention to provide a synchronization system which is effective to maintain synchronization in a TDMA system notwithstanding failure of the reference station sync burst. It is another object of the present invention to provide for synchronization in a TDMA system which does not require a signalling channel dedicated to transmission of information respecting each stations reception of the reference sync burst. A still other object of the present invention is to provide for synchronization in a TDMA system which does not require significant additional hardware in order to maintain synchronization even in the presence of failure of the reference sync burst. A further object of the present invention is to provide for synchronization in a TDMA system which is extremely simple and yet effective to automatically maintain synchronization in the presence of failure of the reference sync burst.